1. Field of the Invention
This invention relates to the field of shift mechanism for manually operated automotive transmissions. More particularly, the invention pertains to a shift mechanism having an interlock to prevent movement of unselected shift forks, a centering assembly to urge the shift shaft to a predetermined neutral position and detents to hold the mechanism in selected positions.
2. Prior Art
U.S. Pat. No. 4,539,859 describes a manual transmission gear shift mechanism in which a selector shaft is forced by a helical compression spring to the gear shift plane in which a shift lever moves whenever the vehicle operator removes manual control over the shift lever. The selector shaft supports a shift finger, which includes detent recesses engaged by a spring-loaded stud, which recesses define positions among which the selector shaft moves. Shift fork rods support shift forks, which include arms having grooves extending into a region in which the shift finger moves. The shift fork arms cooperate with an axially movable locking plate also supported on the selector shaft. When the vehicle operator moves the gear selector lever, the shift finger selects an engagement groove on one of the shift forks. Adjacent the shift finger, the locking plate define locking tongues, which block movement of the shift fork arms that are unselected by engagement with the shift finger. The locking tongues prevent subsequent radial movement of the unselected shift forks as the shift finger moves in response to movement of the selector lever by the vehicle operator.
Production tolerances inherent in this arrangement have a marked effect on the quality of shifting due to the nature of the support of the helical compression springs. This shift mechanism cannot be assembled in the transmission casing in one step; instead, it requires several consecutive assembly steps because the interlocking device is located within the transmission casing.
The shift mechanism of this patent is designed for transverse installation, i.e., for assembly in the motor vehicle whose front wheels are driven by an engine whose crankshaft is directed laterally rather than parallel to the longitudinal axis of the vehicle. The transmission shafts and the shift fork rods that support the shift forks extend transversely with respect to the longitudinal axis of the vehicle and the selector shaft extends substantially horizontally and parallel to the longitudinal axis of the vehicle. The selector shaft is connected to the gear shift lever in the conventional manner using an outer gear shift shaft having conventional joints.
German Patent 24 17 042 describes a transmission connected to an engine arranged transversely in the motor vehicle. The transmission shafts are also arranged transversely with respect to the longitudinal axis of the vehicle. A shift fork rod, also arranged transversely, holds corresponding shift forks, actuated by a vertically directed internal gear shift shaft mounted at its top and bottom in the transmission casing. This shaft is connected by conventional external turning joints to an outer shaft extending substantially horizontal and parallel to the longitudinal axis of the vehicle.
The gear shift mechanism of this patent has the disadvantage that the vertically mounted internal selector shaft located in the transmission casing extends outward beyond the transmission casing at its bottom. The casing must be sealed in that location against the passage of transmission fluid. The stuffing box arrangement present in the device of this patent is extremely complex and expensive. Movement of the selector shaft occurs outside the transmission casing where joints have to be provided in a complex, expensive matter with seals due to the risk of contamination. The difficulties with sealing these several apertures have been solved with the mechanism of the present invention.